Cleaning method for jet engine parts



Dec. 8, 1970 C, L, DQHOGNE ETAL 3,546,084

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United States Patent O T CLEANING METHOD FOR JET ENGINE PARTS Charles L. Dohogne, San Pedro, and James C. Harrington, Cypress, Calif., assignors to Purex Corporation Ltd., Lakewood, Calif., a corporation of California Continuation-impart of application Ser. No. 782,055,

v Dec. 9, 1968. This application May 19, 1969, Ser.

Int. Cl. C23b 1/00 U.S. Cl. 204-141 10 Claims ABSTRACT OF THE DISCLOSURE Metal parts such as vjet engine components having obdurate formations of metal oxides, nitrides, carbides and the like which have often proved irremovable in the past without unacceptable damage to the underlying component are cleaned and polished quickly by a method which sequentially subjects the component to cathodic and anodic treatment at elevated temperatures in strongly alkaline and acidic baths respectively followed by cavitational processing of the treated part and water washing after each bath.

REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our abandoned application Ser. No. 782,055 filed Dec. 9, 1968.

BACKGROUND OF THE INVENTION IField of the invention The modern jet engine has revolutionized air travel in the Aworld and engines of greater size and operating at higher temperatures, thus having improved performance capabilities are on the horizon. Jet engines are formed in their so-called hot section of numerous relatively small components such as turbine blades, discs, vanes and buckets which operate at extremely high temperatures as gases are compressed and the jet fuel burned and the hot gases expelled to provide propulsion. To withstand the mechanical and thermal stresses imposed by jet engine operation these components are formed of specially adapted alloy compositions many of which contain small amounts of'unusual metals for special purposes. In addition, these components are fabricated to precise tolerances requisite to optimum operation.

The combination of hydrocarbon fuel, air and high temperature within the engine produces reaction products of oxygen, carbon, nitrogen, sulfur and other elements with the alloy ingredients forming obdurate scale-like deposits on engine components of unknown constitution which defy removal by conventional metal treating techniques. Moreover, the near zero tolerances for cleaning removal of metal imposed by engine manufacturers pose a difficult dilemma for engine users; on the one hand, it is necessary to remove scale for operating efficiency and safety andon the other hand, cleaning techniques must not be so aggressive as to damage these costly components.

Prior art At present scale removal is accomplished in cleaning baths in the case of easily removed scale and not removed at all or only with protracted effort in the case of obdurate metal oxide, sulfide, nitride and carbide scale which effort may be harmful to the component. For example sand blasting has been used to remove scale after long engine operating periods but this technique is unsatisfactory because much hand labor is required, excessive metal removal occurs, small cracks may be hidden from inspection, abrasive may be retained in complex 3,546,084 Patented Dec. 8, 1970 SUMMARY OF THE INVENTION Accordingly it is a major objective of the present invention to provide a cleaning method which is effective in removal of all manner of metal oxides, suliides, carbides and nitrides and other unwanted metal compounds from multimetallic alloy parts such as components of jet engines and which is elicient to enable rapid removal without damage to the component itself. The present method is fast, with cleaning cycles as short as 20-30 minutes for some components. Moreover it is complete; hidden areas such as faying surfaces between spot welds are cleaned. Also the method is safe; metal removal is negligible.

The method broadly includes sequentially cathodically and anodically treating jet engine components or other parts having surfaces formed of cobalt or nickel-based alloys of surface deposits of metal oxides, metal nitrides, metal carbides and/ or metal sultides and other compounds of alloy metals, by immersing of the component first in a strongly alkaline electroconductive bath at an elevated temperature in which the component forms the cathode and by subsequently immersing the component in a strongly acidic electroconductive bath at an elevated temperature in which the component forms the anode, subjecting the component to ultrasonically cavitating liquid and water washing after each immersing step.

The ultrasonic liquid cavitation may be elected in a water bath and the water washing effected by high pressure sprays following and preceding component immersion in the ultrasonically activated water bath.

The alkaline bath may comprise a fluid mass of sodium hydroxide and potassium hydroxide and water and have a higher temperature than the acidic bath which may comprise a mixture of two or more mineral acids eg. chromic acid and phosphoric acid and sulfuric acid.

During electrochemical cleaning, current densities in the alkaline bath may be greater than those in the acidic bath and the current directionmay be periodically reversed ineither bath.

BRIEF DESCRIPTION OF THE DRAWINGS The single figure depicts schematically preferred sequence of cleaning steps in the present method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While not wishing to be bound to any particular theory of operation, the present effective cleaning method is believed to operate in this manner: The component to be cleaned is placed in a hot alkaline solution which is electroconductive. The component is made cathodic to an electrode, suitably titanium and of a configuration conforming to the component. Hydrogen is evolved from the solution by the current ow. Because the surface metal compounds on the component are nonconductive the hydrogen is evolved as a gas at the metal compound-metal interface; this forces the compound material away from the metal. After a predetermined time the component is removed from solution, rinsed in water and then rinsed in an ultrasonically cavitated liquid to remove any smut (microscopic particles) and trapped oxides from restricted areas, such as narrow passages. The component is then placed in an acid bath and made anodic to a suitable cathode e.g. titanium. This apparently causes metal ions to be removed from the component surface but in such a controlled manner that microscopic high points or asperites are reduced Iwith little or no reduction in relatively recessed surface areas. This tends to polish and debur the part at the same time as removing any remaining smut. This electropolishing reduces the corrosion tendency of the metal component. The component is then water rinsed and again ultrasonically treated to remove retained acid and smut from the now electropolished surface.

The resulting component is clean, as shown by surface resistance tests, even between spot welds. The component may have a matte or reective surface as desired in either case highly corrosion resistant and ideally prepared for inspection. Components have been cleaned in 35 minutes by the herein disclosed method which have been impossible to clean to any measurable degree by the other known methods.

There is no metallurgical damage; components such aS turbine discs and blades may be cleaned without disassembly. Moreover the cleaning cycle may be carried out to differentially clean metal which has been overheated and thus subjected to integranular degradation to provide a visual check means to detect components which should not be returned to service.

In carrying out the present method and with reference to the figure an alkaline bath is prepared comprising one or more alkali metal hydroxide and preferably one part sodium hydroxide, one part potassium hydroxide and one part water at an elevated temperature e.g. 175 275 F. and preferably between 230 and 250 F. The component is immersed in this fluid mass for a brief period e.g. at

between 0.25 and 0.5 amp per square inch. Following electrochemical treatment in the acid bath for several minutes e.g. 2 to 5 minutes the component is removed, spray washed with ambient temperature water and separated from loosened surface deposits by immersion in an elevated temperature water bath at 140-150 F. which is ultrasonically cavitated as above described.

Obviously the time, temperature and current densities may be individually or collectively varied for particular cleaning tasks within externally imposed conditions of metal removal and surface finish desired. It has been found helpful in maintaining cleaning effectiveness to periodically reverse current direction in the two electrochemical baths by changing the electrode polarities for brief periods.

Jet engine components and other parts which may be cleaned of obdurate oxide deposits in accordance with the invention are generally formed of high temperature resistant cobalt and nickel based alloys. The terms cobaltbased and nickel-based herein refer to alloys in which cobalt or nickel, respectively, is the largest single component, in weight percent, although this is not necessarily a major weight portion of the entire alloys. Thus for example suitable cobalt base alloys include those cornposed by weight of cobalt (3S-80%) and tungsten 0- 25%, chromium 040%, iron 0-5%, and/or carbon O- 1.0. Typical alloys are (see chart) line 14. Among nickel base alloys are those composed of nickel -99.5, chromium 0-25, iron 0-25, manganese 0-2, molybdenum 0- 20, cobalt 0-30, silicon 0-2, tungsten 0-15 as well as 0- 20 of columbium, tantalum, aluminum, boron, zirconium, iron and thorium oxide.

PERCENT COMPOSITION Nl C0 M0 W Cb Tl Al Zr Fe Other l Balance.

least 20 minutes to not substantially longer than 60 minutes to soften and condition the surface deposits under electrochemical action generating hydrogen e.g. current densities in the solution between 0.75 and 1.25 and preferably about l amp per square inch.

The component is withdrawn and water washed as with high pressure water spray generally at ambient temperature and then the component is immersed in a bath of water at an elevated temperature e.g. 140150 F. which is ultrasonically cavitated e.g. with energy at 16,000 to 100,000 cycles per second and preferably between 18,000 and 22,000 cycles per second to remove trapped soil and smut from the component with the cavitation.

Following a second spray rinse, the component is immersed as the anode in a previously prepared aqueous mineral acid containing bath and preferably one containing by volume 2 parts aqueous chromic acid solution e.g. 62.5% by weight acid solution, 1 part sulfuric acid e.g. 96% by weight acid solution and 7 parts phosphoric acid e.g. 76% by weight acid solution maintained at a temperature between 135 and 165 F. and preferably 150 to 160 F. to loosen surface deposits by chemical attack and to chemically and electrically polish and brighten the metal. The current density within the acid bath is generally less than the density in the alkaline bath and may range We claim:

1. Method for cleaning parts such as jet engine components having cobalt or nickel-based alloy surfaces of oxides, sulfides, carbides and nitrides of metals of said alloy which includes sequentially cathodically and anodically treating said component, by immersing of the component first in a strongly alkaline electroconductive bath at an elevated temperature in which the component forms the cathode and by subsequently immersing the component in a strongly acidic electroconductive bath at an elevated temperature in which the component forms the anode, subjecting the component to ultrasonically activated liquid and water washing after each immersing step.

2. Method according to claim 1 in which said liquid subjecting includes immersing the component in a water bath which is ultrasonically cavitated.

3. Method according to claim 2 including also water spray washing the component before and after immersion in the ultrasonically cavitated bath.

4. Method according to claim 3 in which the temperature of the alkaline bath is higher than the acid bath temperature.

5. Method according to claim 4 in which the current density in the first bath is greater than the current density in the second bath.

6. Method according to claim 1 or 5 in which the alkaline bath comprises a fluid mass of sodium hydroxide and potassium hydroxide and water.

7. Method according to claim 1 or 5 in which said acidic bath comprises an aqueous mixture of chromic acid, sulfuric acid and phosphoric acid.

8. Method for cleaning parts such as jet engine components having cobalt or nickel based alloy surfaces of oxides and nitrides of metals of said alloy which includes in sequence:

(a) maintaining an alkaline bath comprising by weight one part sodium hydroxide and one part potassium hydroxide and one part water at a temperature between 175 and 275 F.

(b) immersing a component to be cleaned in the alkaline bath (c) passing a current through the bath at a current density of 0.75 to 1.25 amps per square inch with the component functioning as the cathode for a period of not less than 20 minutes nor substantially longer than one hour (d) withdrawing the component from the bath (e) spray washing the component with Water (f) immersing the component in a water bath which is ultrasonically cavitated (g) spray washing the component with water (h) maintaining an aqueous acidic bath containing by volume 2 parts chromic acid solution, 1 part sulfuric acid solution and 7 parts phosphoric acid solution at a temperature between and 165 F.

(i) immersing the component in the acidic bath (j) passing a current through the bath at a current density between 0.25 and 0.5 amp per square inch with the component functioning as the anode for a period of between 2 and 5 minutes, and

(k) repeating steps d, e, f and g.

9. Method according to claim 1 or 8 including also periodically reversing the polarities of the anode and cathode during immersion of the component in the alkaline and acidic baths.

10. Method according to claim 1 or 8 in which the temperature of the ultrasonically cavitated baths is between and 150 F.

References Cited UNITED STATES PATENTS ROBERT K. M'IHALEK, Primary Examiner U.S. Cl. X.R. 204- 

